The invention relates to a concentrate suitable for the preparation of cathodically depositable coating compositions (i.e., cathodic electrodeposition coating compositions) and to the use thereof for the preparation of cathodic electrodeposition (xe2x80x9cCEDxe2x80x9d) coating compositions.
The majority of the CED coating compositions currently employed contain cathodically depositable binders having functional groups comprising active hydrogen, for example hydroxyl groups, primary and/or secondary amino groups, and blocked polyisocyanates as the cross-linking agents. The CED coating compositions are provided as one- or two-component materials, from which cathodic electrodeposition-baths can either be prepared afresh or a compensation of the solids of cathodic electrodeposition-baths in use can be carried out. The one-component materials are water-containing concentrates containing binders and cross-linking agents and, in general, pigments, whereas two-component materials include an aqueous dispersion containing binders and cross-linking agents and a separate aqueous pigment paste.
For reasons of conserving energy, there has been a desire, in particular in the automotive industry, to obtain CED coating compositions that can be stoved at low temperatures.
CED coating compositions that are curable at a low stoving temperature, which contain cathodically depositable binders that are curable with blocked polyisocyanates, and blocked polyisocyanates having isocyanate groups which are bonded to aromatic rings and are blocked with oximes, as the curing agents, are for example known from EP-A-0 199 663, U.S. Pat. Nos. 4,596,744, 4,872,961, 5,096,555 and 5,461,091. The CED materials disclosed therein are chemically unstable. They undergo to a certain extent a decomposition that surprisingly creates no adverse effects in CED coating, but creates problems during transport and storage of the CED materials. During the decomposition, a pressure build-up occurs within the containers containing the CED coating compositions. The speed of the decomposition process depends in particular on the temperature and may lead to an accelerated pressure build-up at high temperatures. The handling of these CED coating compositions in open containers is not allowed under applicable legal restrictions and also for practical reasons. Although the use of pressure containers or pressure compensation containers could be a possible solution for the problem, these CED coating compositions have not been successful, however, in practice because of the requirements involved.
The object of the invention is to provide CED coating compositions which are curable at a low stoving temperature, and which overcome the aforementioned problems.
The present invention provides a two-component concentrate suitable for the preparation of cathodic electrodeposition coating compositions, comprising, in combination, (a) a binder component comprising an aqueous dispersion of a CED binder having groups comprising active hydrogen that are cross-linkable using blocked polyisocyanate; and (b) a cross-linking component comprising an anhydrous, organic solution of an oxime-blocked isocyanate-functional adduct of an aromatic polyisocyanate and at least one compound comprising at least one group capable of addition towards isocyanate and at least one tertiary amino group.
The concentrate according to the invention contains a binder component and a cross-linking component. The binder component is an aqueous dispersion of a CED binder having functional groups comprising active hydrogen that can be chemically cross-linked by using the oxime-blocked isocyanate groups of the cross-linking component.
CED binders carry cationic groups or groups able to be converted into cationic groups, e.g. alkaline groups, e.g. amino groups, ammonium groups, for example quaternary ammonium, phosphonium and/or sulphonium groups. Preferred are alkaline groups, in particular preferred are nitrogen-containing alkaline groups, such as amino groups. These groups may be present in quaternised form, or they are converted into cationic groups with a conventional neutralising agent, for example lactic acid, formic acid, acetic acid, methanesulfonic acid. The groups able to be converted into cationic groups may be present in completely or partially neutralised form. They are preferably resins containing amino groups, preferably tertiary amino groups, the total amine value of which is 20 to 150, preferably 50 to 100 mg KOH/g. The total amine value consists in this case of 50 to 100, preferably 80 to 100, in particular preferably 100% of tertiary amino groups. The weight average molecular mass (Mw) of these binders is preferably about 300 to 10000.
The CED binders contain functional groups comprising active hydrogen that can be cross-linked with the oxime-blocked polyisocyanate. Examples of such functional groups are primary amino groups, secondary amino groups and in particular hydroxyl groups. Combinations of these groups may be present in the same CED binder, but preferably there are no primary or secondary amino groups apart from the hydroxyl groups. The proportion of the groups comprising active hydrogen in the CED binders corresponds to an active hydrogen value of 50 to 300 mg KOH/g, corresponding to a sum of primary amine value, secondary amine value and hydroxyl value. The hydroxyl value is generally in the range of 50 to 250 mg KOH/g.
CED binders suitable for use in the present invention are known to the person skilled in the art and are conventional amino(meth)acrylic resins, aminopolyurethane resins, amino group-containing polybutadiene resins, epoxy resin-carbon dioxydeamine-reaction products and in particular aminoepoxy resins, for example aminoepoxy resins having primary OH-groups.
An aqueous dispersion of the CED binders is prepared according to methods known to those skilled in the art. For example, the CED binders may be neutralised with acid and diluted with water. Organic solvents that may be present can be removed (or eliminated) by vacuum distillation or other known method. It is also possible to prepare the CED binder in a way to minimize the presence of organic solvents, such as neutralising with acid in the low-solvent state or as a solvent-free melt and then diluting with water to make the dispersion. In addition, the CED binders can also be neutralised with acid as a solution in an olefinically unsaturated monomer capable of radical polymerisation, and then be converted into an aqueous dispersion by diluting with water, followed by a subsequent total polymerisation of the monomer capable of radical polymerisation.
The cross-linking component of the concentrate according to the invention is an anhydrous, organic solution of an oxime-blocked isocyanate-functional reaction product of aromatic polyisocyanate and at least one compound comprising at least one group capable of addition towards isocyanate and at least one tertiary amino group. The reaction products blocked with oxime are designated in short as xe2x80x9ccross-linking agentsxe2x80x9d herein.
The cross-linking agents are prepared by reacting one or more aromatic polyisocyanates with at least one compound comprising at least one group capable of addition towards isocyanate and at least one tertiary amino group and one or more oximes (as the blocking agents). The reaction can take place at temperatures of 20 to 75xc2x0 C. as a one-step process or in general as a multi-step process. In a multi-step process, for example, at first an isocyanate-functional reaction product is prepared, the free isocyanate groups of which are blocked with oxime in a subsequent step. Alternatively, at first the aromatic polyisocyanate is partially blocked with oxime prior to the reaction with the further components.
Examples of aromatic polyisocyanates suitable for preparing the cross-linking agents are aromatic diisocyanates (diisocyanates having at least one isocyanate group bonded to an aromatic ring) such as phenylene, toluylene, xylylene or naphthylene diisocyanate and in particular diphenylmethane diisocyanate, each being either in the form of the isomeric mixture or of pure isomers. Further examples are polyisocyanates derived from the aromatic diisocyanates, for example oligomers of the aromatic diisocyanates or isocyanurate or uretdione derivatives of the aromatic diisocyanates or isocyanate-functional adducts of aromatic diisocyanates and polyols or polyamines, such as, for example, trimethylolpropane, 1:1-adducts of dialkanolamines and cyclic carbonate and diethylene triamine.
Examples of compounds comprising at least one group capable of addition towards isocyanate and at least one tertiary amino group are corresponding aminoalcohols, such as dimethyl ethanolamine, dimethyl isopropanolamine, methyl diethanolamine or triethanolamine. Also suitable (and preferred) are polyamines having at least one tertiary amino group and at least one non-tertiary (i.e., primary or secondary) amino group, such as dialkyl aminoalkylamnine, for example diethyl aminopropylamine, dimethyl aminopropylamine and diethyl aminoethylamine.
Examples of oximes suited as the blocking agents are aldoximes or in particular ketoximes such as acetonoxime, cyclohexanonoxime and in particular butanonoxime.
Compounds having at least two groups containing active hydrogen per molecule can optionally be used to prepare the cross-linking agents. Such compounds include polyols, polyamines that contain no tertiary amino groups or aminoalcohols that contain no tertiary amino groups such as ethylene glycol, trimethylolpropane, glycerin, neopentyl glycol, hexanediol, ethylene diamine, diethylene triamine, ethanolamine and methyl ethanolamine.
In a preferred manner, the cross-linking agents comprise 8 to 20 wt-% of isocyanate groups blocked with oxime, calculated as NCO (molecular mass NCO=42), for example corresponding to a number of average 2 to 6 isocyanate groups blocked with oxime per molecule and an amine value of 10 to 50 mg KOH/g. The amine value results in this case from the tertiary amino groups. Hence, the aromatic polyisocyanates, the compounds comprising at least one group capable of addition towards isocyanate and at least one tertiary amino group, the oximes and optionally the compounds having at least two groups containing active hydrogen per molecule are preferably reacted together in such quantity proportions that the cross-linking agents have the aforementioned preferred content of isocyanate groups blocked with oxime and the aforementioned preferred amine value.
The preparation of the cross-linking agents can be carried out without solvents or in the presence of solvent(s) inert towards isocyanate, as the reaction medium. Preferably solvents that are not disturbing or that are even desired in the CED coating compositions, are used. Should an organic solution of the cross-linking agents be prepared which contains solvents that are reactive towards isocyanate, the latter can be added once the synthesis is complete, and any solvents optionally used before as the reaction medium can be removed as carefully as possible, for example by vacuum distillation, until the desired content is achieved.
It is essential to the invention that the cross-linking component is present as an anhydrous organic solution of the cross-linking agents, for example with a solids content of the solution of 50 to 90 wt-%. Examples of suitable organic solvents are solvents conventionally used in CED coating compositions, for example glycol ethers, such as butyl glycol, ethyl glycol, hexyl glycol, methoxypropanol and ethoxypropanol; alcohols, such as butanol and 2-ethylhexanol; ketones, such as methyl isobutylketone, cyclohexanone and methyl ethylketone; propylene carbonate. The cross-linking component is preferably present as a solution in an organic solvent or in a mixture of organic solvents, wherein the solvent (mixture) has a solubility in water of less than 250 g/l water at 20xc2x0 C.
The cross-linking component can contain the cross-linking agents in a non-neutralised form or preferably in a form neutralised with acid, for example corresponding to a neutralisation degree of 50 to 100%. For this purpose, suitable anhydrous acids can be added to the cross-linking agents in corresponding quantities. Examples of anhydrous acids suitable as neutralising agents are the acids conventionally used in CED coating compositions, for example formic acid, acetic acid and methanesulfonic acid.
The concentrate may also contain pigments, fillers and/or additives (collectively xe2x80x9cadditivesxe2x80x9d) that are essential to the preparation of CED coating compositions. These additives may be incorporated into the binder and/or isocyanate components of the concentrate, or may be added to the concentrate as separate components. Such separate components may include, for example, pigment pastes and/or filler pastes. The separate components may be added to the concentrate in any suitable water soluble or water-dilutable form known to those skilled in the art (e.g., as such, as an aqueous or organic solution or as an emulsion, for example).
Examples of pigments and fillers are the conventional inorganic and/or organic coloured pigments and/or effect pigments and/or pigments imparting electrical conductivity and/or fillers. These include, for example, titanium dioxide, iron oxide pigments, carbon black, graphite, phthalocyanine pigments, quinacridone pigments, metallic pigments, interferential pigments, kaolin, talcum, silicon dioxide or corrosion protective pigments. Pigment pastes and/or filler pastes can be prepared by dispersing the pigments and/or fillers in the CED binder or preferably in a paste resin and then ground and then mixed to completion with the outstanding portion of the CED binder or paste resin.
Examples of additives are wetting agents, neutralising agents, levelling agents, catalysts, corrosion inhibitors, defoamers, light protecting agents, antioxidants, radical scavengers and conventional anticratering additives.
The components of the concentrate according to the invention are components which are stable when stored, and which are to be stored separately. They can be stored in conventional, closed containers, such as normal barrels or containers. There is no decomposition with gas generation while the components are stored, and hence, no measures against any pressure build-up are required to be taken in the closed containers.
By mixing the binder component, the cross-linking component and optionally further components of the concentrate according to the invention and by diluting with water, CED coating compositions can be prepared that are curable at a low stoving temperature and that have the desired bath-solids content. The preferred proportion between the binder component and the cross-linking component is in this case 60 to 90 solids parts by weight of CED coating binder: 40 to 10 solids parts by weight cross-linking agent.
The components of the concentrate according to the invention can also be used as refill-materials in the course of a compensation of the solids of CED coating-baths which is required on a regular basis. For this purpose the components are mixed with CED coating-bath material which is poor in solids due to a deposit of CED coating and are then added again to the CED coating-bath.
The CED coating compositions to be prepared by using the concentrate according to the invention are conventional aqueous CED coating compositions having a solids content of for example 10 to 30 wt-%. The solids content consists of the resin solid and any pigments, fillers and non-volatile additives that are present. The resin solids include binders and cross-linking agents of the aforementioned binder and cross-linking component, and optionally paste resins. Apart from water and a content of organic solvents of for example 1.5 to 20 wt-%, based on the resin solids content, the CED coating compositions can also contain volatile additives. The ratio by weight of pigment plus filler/resin solids of the CED coating compositions is for example 0:1 to 0.8:1, preferably between 0.05:1 and 0.4:1 for pigmented CED coating compositions. The content of additives in the CED coating compositions is for example 0.1 to 5 wt-%, based on the resin solids content.
CED coating layers can be deposited from the CED coating compositions in conventional manner on electrically conductive substrates, such as, in particular, automotive bodies, and can be stoved at low stoving temperatures of for example 100 to 150xc2x0 C. object temperature. Higher stoving temperatures of for example up to 200xc2x0 C. object temperature are of course possible.
The CED coating layers can be deposited as an electrically isolating or electrically conductive primer, in the stoved state, on metallic substrates or as an intermediate or top layer on substrates that have been provided with an electrically conductive precoating, for example with an electrically conductive electro-dip primer. The CED coating layers can be stoved separately or together with one or more coating layers that are applied subsequently. Examples of subsequently applied coating layers are coating layers that are applied by spraying, for example primer surfacer, base coat or top coat layers. A subsequently applied coating layer can also be a coating layer that is applied by electrodeposition, if the CED coating layer applied and stoved beforehand has a sufficient electrical conductivity.